Hi and welcome SatDoug.
Gibson never released shipping figures beyond '79. so we don't know exactly how many there are. The silverburst was one of the newer finishes, not available in the 70s. Certainly a rare, but not super-rare finish. They tried a few diffrent colours in the early 80s when they were scaling down production of various models. Candy Apple Red is another possibility (although I can't recall seeing one). The silverburst tends to turn greenish as the clearcoat yellows.
The L-6Ss are all maple - no mahogany (as in an SG) so it has a different tonal feel to it. Brighter, more middish maybe... you'd be best to try it and chose your own adjectives. Yes definately different from both your current guitars.
My L-6S is a Custom, so a little different, but a really nice guitar. $1700 seems a little high, unless this is absolutely mint. Unless you intend not to play it out, i'd wait for a cheaper one to come up. We all know played guitars don't stay mint for long.
If you go for it, please post some pictures!